Reading a tyre size, and what changing it does
How to read a tyre size like 205/55 R16, how sidewall and overall diameter are worked out, what a change does to the speedometer and odometer, and the 3 percent guideline.
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In short
- A tyre size such as 205/55 R16 is a section width in millimetres, a sidewall height as a percentage of that width, and a rim size in inches.
- Overall diameter is rim × 25.4 plus twice the sidewall: 631.9 mm for 205/55 R16.
- A taller tyre covers more road per turn, so the speedometer and odometer read low; a 3 percent larger diameter turns 100 km/h on the dial into about 103.
- Workshops treat a replacement within about 3 percent of the original diameter as acceptable; it is a rule of thumb, not a law.
- A bigger rim with a lower profile can keep the diameter almost unchanged: 225/45 R17 is only 0.4 percent taller than 205/55 R16.
Every tyre carries a string of numbers on its sidewall, and most owners only look at it when it is time to buy a new set. The numbers are a compact description of the tyre's size, and a little arithmetic on them answers the questions that come up at the dealer: will a different size fit, will it make the speedometer wrong, and is the change small enough to be safe? This page explains how to read the code and what the calculator above works out from it.
Reading the size
Take a tyre marked 205/55 R16. The three numbers mean:
| Part | Meaning | For 205/55 R16 |
|---|---|---|
| 205 | Section width in millimetres, from sidewall to sidewall | 205 mm |
| 55 | Aspect ratio: sidewall height as a percentage of the width | 55% of 205 = 112.75 mm |
| R | Radial construction | Radial |
| 16 | Rim diameter in inches | 16 inches = 406.4 mm |
Some tyres also have a letter in front, such as P for a passenger car, and a number and letter after, such as 91V. Those are the load index and the speed rating, which say how much weight the tyre can carry and the top speed it is rated for. They matter when you buy a tyre, but they do not change its size, so the calculator ignores them. It reads 205/55R16, 205/55 R16, 205 55 16 and 205/55R16 91V as the same size.
Working out the diameter
The overall diameter of a tyre is the rim plus a sidewall on each side:
overall diameter = rim × 25.4 + 2 × (width × aspect ratio ÷ 100)
For 205/55 R16 that is 16 × 25.4 + 2 × 112.75 = 406.4 + 225.5 = 631.9 mm. The circumference is π times the diameter, 1,985 mm, so the wheel turns about 504 times to cover a kilometre.
This is the theoretical figure from the size code. A real tyre can differ from it by a few millimetres, depending on the maker, the tread and how much it is inflated, so the actual diameters are in the manufacturers' data. For comparing two sizes, the theoretical diameters are what most workshops use, and any error is nearly the same on both.
Why the diameter matters: the speedometer
A car's speedometer does not measure speed directly; it counts how fast the wheels turn and assumes each turn covers a fixed distance. Fit a tyre with a different diameter and each turn covers a different distance, but the dial is still counting turns.
- A taller tyre covers more road per turn, so the car is faster than the dial says. The speedometer and the odometer both read low.
- A shorter tyre covers less road per turn, so the dial reads high.
The error is the same percentage at every speed. A tyre 3 percent taller makes a dial reading of 80 km/h a true 82.4, and 100 a true 103. For 215/65 R16 in place of 205/55 R16, the diameter goes from 631.9 to 685.9 mm, which is 8.55 percent taller, so 100 km/h on the dial is really 108.6 km/h, and 10,000 km on the odometer is really 10,855 km. A shorter tyre such as 185/70 R14 (614.6 mm, 2.74 percent smaller) does the reverse: 100 on the dial is 97.3 km/h.
Why that matters beyond curiosity: a speedometer that reads 8 percent slow means that driving at what looks like the limit is driving well over it, the odometer under-counts for service intervals, and the electronic systems that compare wheel speeds, such as anti-lock braking and stability control, may be calibrated for a particular tyre size.
The 3 percent guideline
Workshops and tyre sellers commonly say that a replacement tyre's overall diameter should stay within about 3 percent of the original, which for 631.9 mm means between about 613 and 651 mm. Jalopnik's explanation of what it calls the 3% rule describes it as an industry standard for tyre replacements, and its example is a speedometer showing 80 mph while the car is really going 84. The source does not say who set the rule, and it should be taken as a guideline, not a regulation. Your car's maker may specify a narrower range, and in some places a larger change can matter for a vehicle inspection.
The calculator shows the change as a percentage and says whether it is inside 3 percent.
Plus-sizing: bigger rim, same diameter
Many owners fit larger wheels for looks or handling. If the rim goes up by an inch, the sidewall has to get lower by about the same amount to keep the diameter, which is done by lowering the aspect ratio and usually widening the tyre. Compare these with 205/55 R16 (631.9 mm):
| New size | Sidewall | Diameter | Change | 100 on the dial is really |
|---|---|---|---|---|
| 225/45 R17 | 101.3 mm | 634.3 mm | +0.38% | 100.4 km/h |
| 195/65 R15 | 126.8 mm | 634.5 mm | +0.41% | 100.4 km/h |
| 215/60 R16 | 129.0 mm | 664.4 mm | +5.14% | 105.1 km/h |
| 215/65 R16 | 139.8 mm | 685.9 mm | +8.55% | 108.6 km/h |
| 185/70 R14 | 129.5 mm | 614.6 mm | −2.74% | 97.3 km/h |
The first two are almost identical in diameter to the original, one by going up a rim size and the other by going down. The 215/60 R16 is a wider tyre on the same rim, but taller by 5 percent, which is outside the guideline even though the rim has not changed. This is the usual trap: widening a tyre without lowering the aspect ratio makes it taller, because the sidewall is a percentage of a bigger width.
What the calculator does not check
The diameter is only part of whether a tyre suits a car. Check these separately with a dealer:
- Rim width. Each tyre has an approved range of rim widths, and a wide tyre on a narrow rim, or the reverse, does not sit properly.
- Clearance. A wider or taller tyre can rub on the suspension or the body at full steering lock or over bumps.
- Load index and speed rating. A replacement should be at least as high as the original ones.
- Ride and noise. A lower sidewall gives sharper steering and a firmer ride, and rougher roads feel it more.
- Cost. Larger sizes are generally dearer, and low-profile tyres on large rims are easier to damage on poor roads.
Using the calculator
- Enter the size printed on your tyre in the first box.
- Enter the size you are considering in the second.
- Set the speed shown on the dial if you want a different example than 100 km/h.
- Read the percentage change at the top, the true speed under it, and the side-by-side table with the sidewall, diameter, circumference and wheel turns per kilometre.
For other conversions, the Unit Converter handles millimetres and inches, and the Percentage Calculator works out percentage changes. For freight and loading sums, the CBM Calculator is the matching tool.
What this tool assumes
- Sizes are in the metric format, width in millimetres, aspect ratio in percent and rim in inches; sizes with inch widths, such as 31x10.5R15, are not read.
- Diameters come from the size code, not from a manufacturer's data sheet, so they are a few millimetres out in real life.
- The speed error assumes the speedometer was accurate with the original tyres. If yours already reads a little high, the new error adds to that.
- Nothing you type is sent anywhere.